1. Mortality statistics indicate that the addition of a non-nutritive dietary additive (cellulose) markedly increased the life span of middle-aged male mice (16 months). 2. The life span of genetically determined diabetic mice was essentially doubled by feeding diets containing a low level of protein; by reducing the intake of an adequate diet by intermittent feeding; and by adding a non-nutritive additive (cellulose) to an adequate diet. This increased life span was most likely the result of a delay in the onset of the disease as indicated by a marked reduction in polyuria and glucosuria found in the experimental animals. 3. Age and dietary restriction lowers the amount of protein synthesized and degraded by the cell but does not affect these overall processes per se.